Hitherto the valve mechanisms designed for such applications have been of two types: the low-pressure type adapted for actuation by the low-pressure pulses produced by most types of ventilator; and the high-pressure type intended for use with certain ventilators capable of producing appropriately high-pressure pulses. As hitherto designed, valves of the low-pressure type have suffered from the major defect of being designed to develop very small forces to effect their changeover, the valves being therefore delicate and prone to failure as a result of minor damage or distortion of their components by handling -- e.g. during sterilisation procedures -- and, especially, as a result of clogging by foreign matter entering the valve mechanism -- for instance, vomit, etc. exhaled by the patient in emergency situations. On the other hand valves of the high-pressure type, while being generally more robust and less likely to be prevented from operation by the ingress of foreign matter, have the disadvantage of requiring the ventilators with which they may be associated to produce pulses of sufficiently high pressure to effect proper operation of the valves.
An object of the present invention is to provide a valve mechanism that is particularly suited for use as a "patient valve" in conjunction with lung ventilating equipment, and that while actuable by low-pressure pulses derives relatively high force levels from such pulses for actuating its moving elements so as thereby to resist obstruction and impediment by foreign matter that may enter the mechanism.